Children having sex at all is illegal, so legality isn’t the issue
They might ‘know about’ all manner of sexual practices, but do they really need to discuss them at 9? I suppose it comes down to the purpose of the classes. Is it to prepare them for when their bodies change and they start having sexual feelings, or to teach them how to engage in sexual acts? Personally I think that the former approach is healthier, and that encouraging exploration within consensual boundaries will allow them to find out for themselves what they find enjoyable.
I agree that if some children have been exposed to or traumatised by pornography they may want and need to be reassured about how far they will be expected to do the things they have seen - what a world. Is that a job for teachers though? That brings us back to tails wagging dogs, I think.
We, as a society, need to think about how we can allow freedom of choice for adults whilst protecting children from the effects of that. I don’t think that a child can give informed consent to viewing porn (morally as well as legally). When things are seen they can’t be unseen, and shrugging our collective shoulders and saying ‘oh well, they know about these things from the Internet, so we’d better expand sex ed to include them’ doesn’t feel to me to be the best way of dealing with the problem.